The invention is directed to an improved method for reducing sulfur-oxide emissions from an asphalt air-blowing process. More particularly, this invention relates to a method for reducing sulfur-oxide emissions using a filter and an emission-reducing additive. The method has industrial applicability, e.g., in air-blowing asphalt for use as a roofing asphalt or specialty coating.
Although most asphalts are used in paving, a significant percentage are used for other applications, primarily roofing and specialty coatings. Asphalts for roofing and specialty coatings are typically air-blown to reduce aging and to increase resistance to weathering. The air-blowing process also increases the usefulness of the asphalt by raising the softening point from a typical starting point of about 40xc2x0 C. to a softening point of about 80xc2x0 C. or higher.
The air-blowing process involves loading the asphalt raw material into a converter at a temperature of from about 150xc2x0 C. to about 205xc2x0 C. Air is bubbled or blown through the molten asphalt. The reaction produced by the blowing is exothermic and raises the temperature of the asphalt to about 260xc2x0 C. The maximum temperature is usually controlled by a water-cooled jacket. The process is usually carried out in batches. The processing time can take from about 1 hour to about 18 hours to reach the desired softening point. The processing time is dependent on the process temperature, the air flow rate, the characteristics of the asphalt, and the desired product. Catalysts are frequently blended into the mixture to increase the reaction rate and thereby reduce the processing time.
One of the problems with asphalt processing is the generation of undesirable gaseous emissions. The asphalt air-blowing process generates flue gases typically containing hydrogen sulfide, sulfur oxides (SOx), organosulfur compounds, hydrocarbons, nitrogen oxides (NOx), carbon monoxide, and water. Higher throughputs of asphalt in the air-blowing process result in greater amounts of flue gases. Prior to release into the atmosphere, the flue gases are passed through a water-sealed knockout tank and then subjected to an incineration process to control the emissions of volatile organic compounds.
Unfortunately, the incineration process does not control emissions of all the above gases. Sulfur-containing compounds from the asphalt flue gases are oxidized in the incinerator and exit as sulfur-oxide emissions. Typical undesirable sulfur-oxide emissions include sulfur dioxide (SO2) and sulfur trioxide (SO3), and combinations of these compounds with other substances such as water.
Sulfur-oxide compounds are responsible for the generation of acid rain when they condense with humidity. In order to comply with environmental emissions regulations, asphalt-processing plants make use of sound and proven best-available emissions-reduction technology. If the regulated emission levels are not achieved, the asphalt production could be restricted below capacity, or the plant could be forced to pay high fines or penalties.
One process currently used to reduce sulfur-oxide emissions involves cleaning incinerated gases with expensive caustic scrubber equipment. The scrubbers require extensive initial capital costs and significant annual operating costs. The caustic scrubbing process also generates an additional waste stream containing undesirable byproducts, such as sodium and potassium sulfides, sulfites, and sulfates, all of which require special handling for disposal. Other processes are also known for reducing sulfur-oxide emissions, but the processes all suffer from various drawbacks.
Some methods for separating some types of sulfur compounds from certain industrial processes using filters or condensers are known. For example, U.S. Pat. No. 4,915,714 to Teague et al. and U.S. Pat. No. 5,045,094 to Paranjpe disclose fiber filters for use in removing sulfuric-acid mist in an acid-manufacturing process. U.S. Pat. No. 4,741,868 to Rooney et al. discloses fiber filters for use in removing SO3 vapors from a storage tank. U.S. Pat. No. 2,112,250 to Penniman discloses condensers to remove sulfur oxides from a vapor stream formed by oxidizing petroleum. However, there remains a need for an effective method of reducing sulfur-oxide emissions from an asphalt air-blowing process.
One object of the invention is to provide a method for reducing sulfur-oxide emissions from an asphalt air-blowing process without requiring high-cost equipment. Another object is to provide a method that can greatly reduce sulfur-oxide emissions, where the throughput in the air-blowing process is advantageously high while still complying with environmental emissions regulations. A further object is to provide a method for reducing sulfur-oxide emissions prior to the end of the air-blowing process, so that no additional solid or liquid waste stream is created.
These and other objects and advantages have now been achieved through the inventive method for reducing sulfur-oxide emissions from an asphalt air-blowing process. In this method, an emission-reducing additive is combined with asphalt prior to air-blowing, or early in the air-blowing process. The asphalt is subjected to an air-blowing process which produces flue gases including sulfur-containing compounds, and the flue gases are passed through a filter to remove at least a portion of the sulfur-containing compounds. After passing through the filter, the flue gases are subjected to an incineration process before being emitted into the atmosphere. The combination of the filter and emission-reducing additive preferably reduces sulfur-oxide emissions from the asphalt air-blowing process by at least about 50% by weight over the same process without the filter and emission-reducing additive.
The emission-reducing additive is preferably at least one compound selected from metal hydroxides, metal oxides, metal carbonates, metal bicarbonates, and mixtures thereof. Preferred metals are sodium, potassium, calcium, magnesium, zinc, copper, aluminum, and mixtures thereof. An especially preferred additive contains an alkali-metal hydroxide and zinc and copper oxides.
The filter or phase-change element is preferably a fibrous or fabric material, such as a packing of glass or polymer fibers. A preferred fibrous filter material has a packed fiber density of from about 130 kilograms/meter3 to about 320 kilograms/meter3. Preferably, the ratio of the gas flow rate in cubic meters per minute to filter surface area in square meters is from about 0.9 to about 9, more preferably from 1.5 to 4.6.
One embodiment of the inventive method for reducing sulfur-oxide emissions from an asphalt blowing process comprises: adding to an asphalt an emission-reducing additive comprising a metal hydroxide, metal oxide, metal carbonate, and/or metal bicarbonate, wherein the metal is selected from sodium, potassium, calcium, magnesium, zinc, copper, and aluminum, to form an asphalt-additive mixture; subjecting the asphalt-additive mixture to a blowing process that produces a gas stream including sulfur-containing compounds; passing the gas stream through a fiber filter to remove at least a portion of the sulfur-containing compounds, wherein the filter comprises fibers packed at a density of from about 130 kilograms/meter3 to about 320 kilograms/meter3, the ratio of the flow rate of the gas stream through the filter to the surface area of the filter is from about 0.9 meter3/minute-meter2 to about 9 meters3/minute-meter2, and the gas stream is at a temperature not greater than about 121xc2x0 C. when passed through the filter; and subjecting the filtered gas stream to an incineration process. Another embodiment includes the steps of: adding to an asphalt an emission-reducing additive in an amount of from about 0.1% to about 2.0% by weight of the asphalt combined with the emission-reducing additive, the emission-reducing additive comprising at least one metal hydroxide selected from sodium hydroxide, potassium hydroxide, calcium hydroxide, and magnesium hydroxide, and at least one metal oxide selected from zinc oxide, copper oxide, and aluminum oxide; subjecting the asphalt to an air-blowing process that produces a gas stream including sulfur-containing compounds; passing the gas stream through a filter to condense and remove at least a portion of the sulfur-containing compounds; and subjecting the filtered gas stream to an incineration process.
Other embodiments and features of the invention will become apparent from the detailed description in conjunction with the drawing figures.